1. Field of the Invention
This invention relates to high-temperature heat exchangers for gas streams. More specifically, improved apparatus and method for recovering heat from the furnace effluents stream in a carbon black plant to preheat the combustion air stream are provided.
2. Description of Related Art
In the typical carbon black production process, fuel and air are combusted in a furnace to provide the necessary temperature and energy for the carbon black production step. Oil feedstock is injected directly into the combustion gases, still inside the furnace, where the feedstock is dehydrogenated in a pyrolytic reaction to form carbon black and other gaseous products. The final stream, after all the reactions are complete, is referred to as "smoke." In order to completely stop all the reactions and to cool the furnace effluent, the smoke is quenched by direct contact with water. After the water quench, the smoke stream is still very hot and can be used to heat other process streams, such as the combustion air stream.
Preheating the combustion air stream significantly increases efficiency of the carbon black production process by reducing the amount of fuel required while also increasing the capacity of a carbon black production unit. Various processes and apparatuses for preheating the combustion air stream in a carbon black production process are known to the industry. Most carbon black production processes use a vertical shell-and-tube heat exchanger to preheat the combustion air stream by indirect contact with the smoke stream exiting the water quench. The smoke stream typically flows upwards through the tubes while the air stream is forced downward through the shell. Combustion air exiting the current industry standard air preheater may be heated to temperatures up to about 800.degree. C.
Several problems must be considered when designing a preheater for carbon black production. First is the tendency of the smoke stream to deposit carbon particles inside the tubes, thus fouling those surfaces and reducing heat exchange efficiency. If the smoke stream is cooled too much, the fouling becomes particularly pronounced. Thus, most preheating processes and apparatuses are designed to keep the smoke stream hot in order to reduce fouling as much as possible.
Several remedies exist in current practice to maintain the smoke streams at high temperatures and prevent fouling. First, a sheath may be constructed around the top portion of the tubes, thus creating a stagnant air gap between the sheath and the tube surface. The air gap reduces heat exchange in the area of the sheath and thus reduces cooling of the smoke stream. Unfortunately, the amount of heat transferred to the combustion air is also reduced, resulting in a less efficient preheater. Furthermore, the sheathing complicates the heat exchanger manufacturing process and adds to exchanger cost.
A second remedy is to decrease flow of combustion air through the preheater. U.S. Pat. No. 4,737,531 discloses a method by which a control valve causes a fraction of combustion air to bypass the preheater. A lower flow of combustion air through the preheater transfers less heat away from the smoke stream, keeping the smoke at a temperature higher than the temperature at which high rates of fouling occur. This method requires a complicated and costly control system and preheats only a portion of the combustion air.
A third design uses a double tubesheet (two parallel, closely spaced, tubesheets) in a heat exchanger and two stages of air compression ("Improvements to High Temperature Airheater," presented at Carbon Black World 96, Nice, France, Mar. 4-6, 1996). Hot gas from a reactor, carrying carbon black smoke, is passed through the tubes of a shell-and-tube heat exchanger. The double tubesheet in the shell around the inflow end of the tubes creates two separate heat exchange compartments on the shell side. Compressed air is fed to the air pre-heater from a first compression stage. About 20% of the air stream from the first compression stage is diverted to a second compression stage and forced between the double tubesheets, which form a small compartment on the shell side of the exchanger. Air flows radially inward across the tubes between the double tubesheets and is then directed up a center tube in the shell to the top of the heat exchanger. The preheated air then encounters a baffle system at the top of the heat exchanger where it mixes with compressed unheated air from the first stage of compression. The combined stream then flows through the shell side countercurrent to flow of the smoke stream in the tubes.
The slip stream that is further compressed and sent to the top of the tubes serves to increase temperature of the top of the tubes, which reduces fouling in the top section of the tubes, but shielding of the top section of the tubes is still normally required to prevent fouling. Shielding of the tubes, which decreases heating of the incoming air, causes loss of efficiency of the pre-heating process, as discussed above.
The double tubesheet at the end where the hot smoke stream enters the pre-heater addresses another problem of combustion air preheaters--mechanical failure of the tubes and the tubesheet caused by high temperature of the smoke stream. Cool air transfers heat away from the tubes and tubesheets in the entry zone and reduces thermal stress on the heat exchanger. Without the double tubesheet, lower temperature of the incoming stream and insulation in the tubes to decrease heat transfer rate are necessary, both of which cause loss of efficiency.
One additional drawback of the third design is a limitation of the volume available between the double tubesheets, and thus a limitation of the temperature that can be attained in the air stream that is to be directed into the shell at the outflow end of the tubes. The heat exchanger tubes are welded to a sleeve which is welded to the middle tubesheet and lower tubesheet, and the tubesheets are welded to the shell in this design. This results in the elimination of air leakage from across the center tubesheet, but it causes other problems. The tubes expand during operation due to their increased temperature. This expansion places stress on the sleeves and tubesheets, and the stress may cause failure, especially at the point where the sleeves are welded to the tubesheet. The greater the distance between the tubesheets, the more stress is created. Limitations in the amount of stress that the tubesheets can tolerate restrict the distance between the tubesheets in the prior art design. Thus, the maximum volume between the double tubesheets and the maximum flow through that compartment of the shell is restricted.
While the third design makes improvements in the operation of air preheaters for use in carbon black plants, increased complexity in design of the heat exchanger and increased cost of a second compression step are necessary. Shielding of the top of the tubes may still decrease efficiency.
All of the above mentioned high temperature air preheater designs utilize expansion joints which are welded to the upper tubesheet. Commercially available expansion joints have a significantly larger diameter than the tubes, and thus allow for little tubesheet material between tubes in the upper tubesheet. Also, the expansion joints put extra stress on the bottom and top tubesheets. The combination of little top tubesheet material and stress often causes tubesheet failure. Further, the commercially available expansion joints themselves are often prone to fail.
What is needed is a heat exchanger system that has reduced complexity and cost while retaining and improving efficiency of the heat recovery process and increasing service life of the system.